1. Field of the Invention
The present invention generally concerns the acoustic measurement of fluid flow velocity within a lumen; particularly the in-vivo measurement of blood flow in a blood vessel by a transit time acoustic flowmeter.
The present invention particularly concerns a spatial, and an electrical, configuration of multiple transducers within a transit time acoustic flowmeter in order that the size of an acoustic probe may be minimized in both length and diameter-while the acoustic path over which acoustic propagation delay is measured is maximized.
2. Description of the Prior Art
2.1 General Operation of Acoustic Flowmeters and Associated Sonic Transducers
The transit time of sound projected diagonally across a flowing fluid has been used to measure the rate of the fluid's flow. The flowing fluid is normally contained in a conduit, or lumen. In various forms of transit time and Doppler type acoustic flowmeters one or more sonic transducers are used. In one common form of a transit time acoustic flowmeter two transducers are positioned against a lumen flowing fluid and diagonally across the lumen from each other. One transducer is located in a relatively more upstream, and the other in a relatively more downstream, position. Sound is transmitted through the flowing fluid between the transducers in each of the Upstream and downstream directions.
The bi-directional acoustic signals are variably delayed by their passage through the flowing fluid. Measurement of difference between the acoustic signal delay, or phase shift, in the upstream and downstream sound propagation directions is indicative of the velocity of fluid flow in the lumen between the transducers. Previous transit time acoustic flowmeters have used various acoustic modulation and demodulation schemes in combination with continuous, or pulsed, ultrasound.
In previous transit time flowmeters acoustic signals have either been alternately or simultaneously transmitted along opposing paths between one or more pairs of transducers. Most commonly transit time flowmeters send acoustic signals alternately in the upstream (obliquely across a lumen flowing fluid) and downstream directions. More rarely, some transit time flowmeters send an acoustic signal upstream between the transducers of one transducer pair simultaneously that another acoustic signal is sent downstream between the transducers of another transducer pair. The previous transit time flowmeter employing multiple transducer pairs, and multiple acoustic paths between the transducers, is most closely analogous to the present invention.
However, the present invention will be seen to function differently from previous transit time flowmeters using multiple pairs of transducers in that an acoustic signal will be sent between the transducers of two successive transducer pairs in the same--as opposed to opposite--upstream/downstream direction. The propagation on the two successive acoustic paths will be seen to be successive in that propagation will not start upon a second acoustic path until it has previously commenced upon a first. acoustic path. However, the propagation on the two successive acoustic paths will also be seen to be simultaneous in that an acoustic signal will typically sequentially commence propagation upon the second acoustic path so quickly that a first acoustic signal will still, simultaneously, be in progress upon a first acoustic path. As is typical, and even fundamental, to transit time flowmeters, the acoustic signals on both acoustic paths of the present invention will be seen to, at a later time, pass between the same pairs of transducers in the reverse direction. This later passage of acoustic signals will again be--collectively as between the transducers of all transducer pairs--in the same upstream/downstream direction. Accordingly, and notably, in the present invention an acoustic signal between the two transducers of each of a number (typically two) of transducer pairs will be seen to propagate both (i) sequentially and (ii) simultaneously in a same--as opposed to an opposite--direction relative to the upstream-downstream direction of the flowing fluid.
2.2 Full Acoustic Illumination of the Flowing Fluid by the Transducers
Some previous acoustic flowmeters used small transducers which only served to acoustically illuminate only a central portion of fluid flowing within a lumen. The velocity of the flowing fluid was measured at, and for, only the acoustically illuminated (central) portion of the lumen. Fluid flow within a lumen is typically laminar, and non-turbulent, with a stratified profile of flow velocity. In such a stratified profile of flow velocity the fluid flows fastest at the center of the lumen and slowest along the lumen's wall(s). Accordingly, a measurement of the velocity of fluid flow only at the central portion of the lumen, howsoever accurate, cannot, by definition, be truly indicate of the average velocity of fluid flow throughout the entire cross-sectional area of the lumen.
Improvements have been made to acoustic flowmeters in order to more correctly measure the flow of fluid within a lumen when the fluid is flowing at a complex profile of flow velocity. Reference U.S. Pat. Nos. 3,575,050 for a "Fluid Flowmeter" and 3,906,791 for an "Area Averaging Ultrasonic Flowmeter" to Lynnworth. One improvement was to use wide transducers, normally so wide so as to evenly acoustically illuminate the entire cross section of fluid flowing within a lumen. By this method the velocity of fluid flow could be area averaged over all, or substantially all, of the lumen's cross-sectional area, and a the average fluid flow velocity within the lumen could be more accurately determined. The present invention will be seen to function for full acoustic illumination of the fluid flowing within the lumen--as is desirable--while functioning to do so in a particularly compact geometry.
2.3 Sensitivity of Acoustic Flow Measurement to Alignment of the Transducers With the Flow Axis
Acoustic fluid flow measurement systems developed and shown by Lynnworth in U.S. Pat. No. 3,906,791, by Drost in U.S. Pat. No. 4,227,407, and by others place both transducers on the same side of a conduit and use a sound reflecting surface, or reflecting-type transducer, to reflect the sound beam off the far wall of the conduit. This configuration is widely used for the measurement of fluid flow through closed channels in industrial applications. It has been shown that the reflecting technique makes a first order correction for alignment errors under conditions where the flow axis of the conduit is not aligned with the transducer array axis and the flow conduit is substantially smaller than the transducer array. The sound reflector is rather long and oddly shaped which, combined with its weight, makes it disadvantageous for implantation within a living animal for blood flow measurement applications, especially those persisting over a long term.
The present invention will be seen to enjoy the same insensitivity to alignment between the transducers and the flow axis as is realized in the prior art without the necessity, and attendant size and weight penalty, of using a sound reflector.